Fix deferring free object that refcount is more than 1 (#14738)

in https://github.com/redis/redis/pull/14440, we remove the refcount
check in
[tryDeferFreeClientObject](235e688b01 (diff-252bce0cc340542712f0c1adf62e9035ea47a4a064321fbf40ec3dd4b814aaf2R1509)),
it is ok in 8.4 version, since after command execution, the refcount of
a kvobject always is 1.
but in #14608 (8.6 RC1) we change this assumption, increment refcount
when a client refer a kvobject in reply, so now if the refcount of
kvobject is more than 1, we may let the io thread call `decrRefCount`,
there is data race, maybe it causes memory leak.

src/db.c

@@ -687,7 +687,7 @@ static void dbSetValue(redisDb *db, robj *key, robj **valref, dictEntryLink link
         }
     }
 
-    if (server.io_threads_num > 1 && old->encoding == OBJ_ENCODING_RAW) {
+    if (server.io_threads_num > 1 && old->encoding == OBJ_ENCODING_RAW && old->refcount == 1) {
         /* In multi-threaded mode, the OBJ_ENCODING_RAW string object usually is
          * allocated in the IO thread, so we defer the free to the IO thread.
          * Besides, we never free a string object in BIO threads, so, even with